The invention related to an apparatus for the pretreatment and subsequent conveying, plastification, or agglomeration of plastics.
The prior art reveals numerous similar apparatuses of varying design, comprising a receiver or cutter compactor for the comminution, heating, softening and treatment of a plastics material to be recycled, and also, attached thereto, a conveyor or extruder for the melting of the material thus prepared. The aim here is to obtain a final product of the highest possible quality, mostly in the form of pellets.
By way of example, EP 123 771 or EP 303 929 describe apparatuses with a receiver (receiver container) and, attached thereto, an extruder, where the plastics material introduced into the receiver is comminuted through rotation of the comminution and mixing implements and is fluidized, and is simultaneously heated by the energy introduced. A mixture with sufficiently good thermal homogeneity is thus formed. This mixture is discharged after an appropriate residence time from the receiver into the screw-based extruder, and is conveyed and, during this process, plastified or melted. The arrangement here has the screw-based extruder approximately at the level of the comminution implements. The softened plastics particles are thus actively forced or stuffed into the extruder by the mixing implements.
It is also known in principle that twin-screw extruders can be used and that these can be linked to appropriate cutter compactors.
Many of these designs, which have been known for a long time, however, are unsatisfactory in respect of the quality of the treated plastics material obtained at the outgoing end of the screw, and/or in respect of the throughput of the screw. Especially when twin screws are used, there are particular considerations which cannot be derived from the results for single screws.
A distinction can be made between co-rotating and counter-rotating, and also touching and tightly intermeshing, twin-screw conveyors or twin-screw extruders, as a function of the axial distance therein between the screws and of their relative direction of rotation.
In the case of co-rotating screws, the two screws rotate in the same direction with identical angular velocities.
Each of these types has particular application sectors and uses. In the case of the co-rotating twin-screw extruder, the conveying and the pressure increase are brought about in essence by virtue of the friction between the stationary housing wall and the material rotating concomitantly with the screw, and the conveying effect is mainly a result of drag flow. In the case of the counter-rotating twin-screw extruder, in contrast, the dominant principle is forced conveying.
Critical to the end quality of the product are, firstly, the quality of the pretreated or softened polymer material that enters the conveyor or extruder from the cutter compactor, and, additionally, the situation at intake and on conveying or, where appropriate, extrusion. Relevant factors here include the length of the individual regions or zones of the screw, and also the screw parameters, such as, for example, screw thickness, flight depths, and so on.
In the case of the present cutter compactor/conveyor combinations, accordingly, there are particular circumstances, since the material which enters the conveyor is not introduced directly, without treatment and cold, but instead has already been pretreated in the cutter compactor, viz. heated, softened and/or partly crystallized, etc. This is a co-determining factor for the intake and for the quality of the material.
The two systems—that is, the cutter compactor and the conveyor—exert an influence on one another, and the outcomes of the intake and of the further conveying, and compaction, where appropriate, are heavily dependent on the pretreatment and the consistency of the material.
One important region, accordingly, is the interface between the cutter compactor and the conveyor, in other words the region where the homogenized pretreated material is passed from the cutter compactor into the conveyor or extruder. On the one hand, this is a purely mechanical problem area, requiring the coupling to one another of two differently operating devices. Moreover, this interface is tricky for the polymer material as well, since at this point the material is usually, close to the melting range, in a highly softened state, but is not allowed to melt. If the temperature is too low, then there are falls in the throughput and the quality; if the temperature is too high, and if unwanted melting occurs at certain places, then the intake becomes blocked.
Furthermore, precise metering and feeding of the conveyor is difficult, since the system is a closed system and there is no direct access to the intake; instead, the feeding of the material takes place from the cutter compactor, and therefore cannot be influenced directly, via a gravimetric metering device, for example.
It is therefore critical to design this transition not only in a mechanically considered way, in other words with an understanding of the polymer properties, but at the same time to consider the economics of the overall operation—in other words, high throughput and appropriate quality. The preconditions to be observed here are in some cases mutually contradictory.
Co-rotating multi- or twin-screw conveyors must generally be operated with underfeed. With underfeed, the feed determines the throughput of the extruder, and material intake has to be very constant.
However, specifically in systems where there is a conveyor or extruder attached to a cutter compactor, the intake or feed into the twin-screw conveyor is anything but easy to adjust, and there is no possibility, for example, of metering by way of a gravimetric metering system. On the contrary, in a cutter compactor the rotating mixing and comminution implements bring about continuous feed of the pretreated, softened particles or a continuous flow of material to the intake aperture of the conveyor or extruder.
In addition, another factor common to the known apparatuses is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver, and the direction of conveying of the conveyor, in particular an extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder or conveyor, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder or of the screw. This firstly reduces the conveying effect of the screw, and secondly there can also be some reverse flow of the said melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage of the intake and caking of the mixing and comminution implements. A further consequence is reduction of the throughput or quantitative output of the conveyor or extruder, since adequate filling of the screw is no longer achieved. Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction.
Especially for co-rotating twin-screw conveyors, this type of stuffing effect is counter-productive, and it is very difficult to avoid overfeed.